Gauge-singlet pseudoscalar particles have been attracting attention for many years in view of understanding the phenomenology of the strong CP
problem (lack of CP violation in QCD) [1]}, [2]}, [3]}. Such particles appear in models
with a spontaneously broken global symmetry and are considered as candidates for either dark matter or for mediators to a dark sector, see, e.g.,
Refs. [4]}, [5]}, [6]}, [7]}, [8]}, [9]}, [10]}, [11]}.
